One type of process heat exchanger used in various industries comprises a tank having an inlet and an outlet for the liquid to flow through. One or more heater element bundles are mounted in the tank. Each heater element bundle has a number of electrical resistance heater elements. Each heater element includes a metal tube, an electrical resistance coil within the tube and embedded in an insulation powder, and a conductor pin secured to the end of the coil. The heater element bundle has at least one end located outside of the tank, that end having a header to which each tube is secured. The opposite end may also include a header or the tubes may be U-shaped.
Voltage is applied to the conductor pins to create heat in the electrical resistance coils. Most process heat exchangers operate with three-phase power in the range from about 600 to 640 volts. More recently heat exchangers have been proposed to operate in the range from about 2,400 to 4,160 volts. Although considered a medium voltage for electrical power transmission in general, this voltage creates more demands on the heat exchanger heater elements.
The insulation powder is typically magnesium oxide packed tightly within the tube surrounding the coiled wire. While magnesium oxide provides excellent electrical insulation, it is a desiccant, thus it attracts moisture from the surrounding atmosphere. The penetration of moisture reduces the ability of the insulation powder to insulate.
In the past, heater elements of this nature have been kept in low humidity rooms and/or baked in an oven with their ends open to drive off any moisture. Then, when ready for use, the heater element is mounted to a header plate and seals are placed over the open ends. For example, a liquid sealant may be poured over the open ends and cured. While these methods work, improving the resistance of the insulation is desirable not only for low voltage process heat exchanger elements but particularly for medium voltage heater elements. The higher voltage is more difficult to insulate, particularly at the exposed end face of the insulation powder, which is subject to moisture penetration.